Optical disc drives, such as CD-ROMs, are commonly used for storing large amounts of digital data on a single disc for use in audio/video or computer applications, and the like. The data on an optical disc is typically recorded as a series of "pits" arranged in tracks, where the length of the pit determines the presence of a digital "0" bit or a "1" bit. To read this recorded data, a servo system focuses a laser beam onto the surface of the disc such that the characteristics of the reflected beam allow detection of the data pits.
To this end, the servo system performs four operations: (1) a capture operation to "pull-in" the initial focus position, (2) a seek operation to move the beam to a desired track, (3) a centerline tracking operation to maintain the beam over the centerline of the selected track while reading the recorded data, and (4) a focus tracking operation to maintain proper focus as the disc spins over the beam.
Conventional optical disc drives use a head assembly comprised of a laser diode for generating the laser beam which is focused onto the surface of the optical disc through an objective lens. FIG. 1A illustrates a typical three beam optical head assembly, the operation of which is well known by those skilled in the art. A laser diode 1 produces a light beam which passes through a diffraction grating (not shown) splitting the main beam into three separate beams 2 (a center beam and two side beams); the three beams 2 then pass through a polarization beam splitter 3 and a collimator lens (not shown). The light beams 2 are then reflected by a prism 4, through an object lens (OL) 5, and onto the surface of the optical disc (not shown). The beams 2 reflect off the optical disc, again pass through the OL 5, and then reflect off the prism 4 back toward the polarization prism 3. The polarization prism 3 deflects the center beam onto a quadrant photodetector 6, and deflects the two side beams onto two tracking photodiodes (7A, 7B). The quadrant photodetector 6 generates a focus error signal (FES) for focusing the OL 5, and it generates an RF read signal for reading the recorded data. The tracking photodiodes (7A,7B) generate a tracking error signal (TES) used to maintain the position of the OL 5 over the centerline of a selected track while reading data from the disc.
In order to position the read head over a selected track during a seek operation, the entire sled assembly 8 slides radially along a lead screw 9 underneath the optical disc until the read head is positioned near the desired track. This coarse positioning (or coarse seeking) is accomplished by rotating the lead screw 9 in a clockwise or counterclockwise direction. Once near the selected track, OL voice coil motors (VCMs) (10A,10B) rotate an OL carriage unit 11 about a plastic hinge 12 in a "fine seeking" operation until the OL 5 is positioned directly over the desired track. Then, as the disc rotates and the track passes under the read head, the OL VCMs (10A,10B) perform fine adjustments in a "tracking" operation in order to maintain the position of the OL 5 over the centerline of the selected track as information is read from the disc.
The OL VCMs (10A,10B) can rotate the OL carriage unit 11, and thereby the OL 5, about the plastic hinge 12 in a range that spans approximately 200 tracks on either side of its center position. Thus, if during a seek operation the selected track is within 200 tracks of the current track, the OL carriage unit 11 can perform the entire seek operation without needing to slide the sled assembly 8 along the lead screw 9. At the end of a short seek operation and while tracking the centerline of the selected track, the lead screw 9 slowly slides the sled assembly 8 toward the selected track until the OL carriage unit 11 is again in its center position.
The OL VCMs (10A,10B) also move the OL carriage unit 11 up and down in the direction shown in order to "capture" and "track" the OL 5 focus position. For focus capture and focus tracking the quadrant photodetector 6 generates an astigmatic focus error signal indicative of the distance between the OL 5 and the optical disc. At the beginning of a capture operation, the OL carriage unit 1 is initially positioned sufficiently away from the disc so that it is out-of-focus. Then the OL VCMs (10A,10B) slowly move the OL carriage unit 11 toward the disc with the focus servo loop open until the quadrant photodetector 6 indicates that the OL 5 is within its focus pull-in range. Once within the pull-in range, the focus servo loop is closed and the initial focus point is captured. Thereafter, the OL VCMs (10A,10B) track the in-focus position in response to the astigmatic focus error signal (FES) as the read head seeks to selected tracks and reads data from the disc.
FIG. 1B illustrates how the changing image on the quadrant photodetector 6 generates the focus error signal (FES). When the disc surface lies precisely at the focal point of the OL 5, a circular spot strikes the center of the quadrant photodetector 6. When the distance between the disc and the OL 5 decreases, the reflected image becomes elliptical. Similarly, when the distance between the disc and the OL 5 increases, an elliptical pattern again results, only rotated 90 degrees from the first elliptical pattern. Thus, as shown in FIG. 1C, the quadrant photodetector 6 generates the focus error signal (FES) according to (A+C)-(B+D), and it generates the RF read signal according to (A+B+C+D). FIG. 1D is a plot of the focus error signal (FES) versus the distance between the OL 5 and the optical disc, where the linear region defines the focus "pull-in" range for the focus capture operation.
Similar to the quadrant photodetector 6, the tracking photodiodes (7A,7B) generate a tracking error signal (TES) used by the servo control system to maintain the OL 5 over the centerline of the selected track as the disc spins over the beam. FIG. 1E shows how the tracking photodiodes (7A,7B) generate the tracking error signal (TES) according to the intensity of the side beams. When positioned perfectly over the track's centerline, the tracking error signal is zero. When positioned to the left or right of the centerline, the tracking error signal (TES) is positive or negative, respectively. Thus, the tracking error signal (TES) is generated as (E-F) as illustrated by FIG. 1F. FIG. 1G is a plot of the tracking error signal (TES) versus the mistracking, where the linear region indicates the pull-in region for the tracking operation.
There are many difficulties inherent in controlling the read head actuator of an optical disc drive relative to focus capture, focus tracking, track seeking and centerline tracking. One significant aspect is the wide range of parametric variations that can occur due to internal factors, such as temperature and volt drift, as well as parametric variations in the servo system between disc drives. The servo system must be able to compensate for these parametric variations in order to perform adequately at the ever increasing data rates and densities.
Yet another problem is that the above described capture operation does not always end successfully; rather it can fail due to over shooting the pull-in range as defined by the linear region of the focus error signal (FES) shown in FIG. 1D. That is, there is a significant capture transient associated with closing the focus servo loop (see FIG. 14A) that varies with such factors as the relative disk/head velocity variation, surface contaminations and the driving force of the focus VCMs. For this reason, optical storage devices are generally designed to repeat the capture operation several times.
Still another problem is maintaining focus during seek operations which presents a significant external disturbance to the focus servo loop. If the focus is lost during a seek, the storage system must pause to perform a focus capture operation which can significantly increase the seek time.
Yet another problem associated with optical disc servo systems is the optical coupling or feed through phenomena that occurs between the focus tracking and centerline tracking loops. U.S. Pat. No. 5,367,513 discloses one solution to the optical feedthrough problem, but it has disadvantages which are overcome by the present invention--mainly, cost of implementation.
Conventional servo systems found in optical storage systems typically implement a linear controller using Proportional-Integral-Derivative (PID) feedback and/or state estimators. The problem with conventional linear controllers, however, is they are sensitive to parametric variations in the servo system and to external load disturbances. Conventional adaptive linear controllers overcome this sensitivity problem by executing complex calibration routines or by continuously re-programming the controller to compensate for the parameter variations and load disturbances. However, adaptive linear controllers are complex and may require notch filters for filtering out mechanical resonances. Furthermore, conventional linear controllers are not adept to the above described problems inherent in controlling an actuator for an optical disc drive.
There is, therefore, a need for an optical disc drive servo control system that is less sensitive to parametric variations, provides better control of transients, and avoids the implementation cost of a complex, adaptive linear controller.